1. Field of the Invention
The present invention relates to a piezoelectric resonator. In particular, the present invention relates to a piezoelectric resonator including an acoustic reflector portion to acoustically separate a vibration portion, which includes a piezoelectric thin film sandwiched between a pair of electrodes, from a substrate and to support the vibration portion.
2. Description of the Related Art
Previously, piezoelectric resonators (Bulk Acoustic Wave resonators or BAW resonators) taking advantage of thickness vibration of a piezoelectric thin film and piezoelectric filters formed from the piezoelectric resonators include a type provided with a cavity and a type provided with an acoustic reflector portion to acoustically separate a vibration portion, which includes a piezoelectric thin film sandwiched between a pair of electrodes, from a substrate and to support the vibration portion.
Regarding the latter type, as in a piezoelectric resonator 100 indicated by a sectional view shown in FIG. 8, an acoustic reflector portion 122 is disposed between a vibration portion 120, in which a piezoelectric thin film 116 is sandwiched between a pair of electrodes 115 and 117, and a substrate 112. Regarding the acoustic reflector portion 122, low acoustic impedance layers 114a and 114b formed from a material having relatively low acoustic impedance and high acoustic impedance layers 113a and 113b formed from a material having relatively high acoustic impedance are laminated alternately. The acoustic reflector portion 122 reflects vibration from the vibration portion 120 and acoustically separates the vibration portion 120 from the substrate 112 in such a way that the vibration is not transferred to the substrate 112. For example, a metal material (W, Mo) is used for a high acoustic impedance layer, and a dielectric material (SiO2, Si3N4) is used for a low acoustic impedance layer (refer to Japanese Unexamined Patent Application Publication No. 2002-251190, for example).
Furthermore, as is indicated by a sectional view shown in FIG. 9, the document Proceedings of The 8th International Symposium on Sputtering and Plasma Process, “Properties of Sputter Deposited AlN, Mo, W and SiO2 Thin-Films for Bulk-Acoustic-Wave Applications on 200 mm Si Substrates” discloses that SiO2 is used for a low acoustic impedance layer, W is used for a high acoustic impedance layer, and AlN is used for a seed layer.
Regarding the piezoelectric resonator of Japanese Unexamined Patent Application Publication No. 2002-251190, in the case where the metal (for example, W) serving as the high acoustic impedance layer is etched, there is a possibility that the low acoustic impedance layer thereunder is also etched. If a height difference formed around a portion, in which the electrodes are opposed to each other, of the vibration portion increases because of such over etching, a break in the electrode occurs easily. Furthermore, in the case where the film thickness of the high acoustic impedance layer is large, the risk of peeling of the film and cracking of the substrate increases. Consequently, it is difficult to conduct production with stable quality.
Regarding the publication “Properties of Sputter Deposited AlN, Mo, W and SiO2 Thin-Films for Bulk-Acoustic-Wave Applications on 200 mm Si Substrates,” the W film is specified to have a tensile stress for the purpose of stress adjustment of the acoustic reflector portion. The W film having the tensile stress exhibits poor crystallinity and tends to become a non-dense film, so that the resonance characteristics are degraded.